The feathered variety.
My boat is permanantly afloat and parked in a marina. Every time I go to see it, a load of birds have pooped all over it. well, at least I think it's bird poo. I won't go into details....lets just say it's a problem.
What can be done?
Apart from the appearance, there is the thought that the stuff may be corroding parts of the boat as well. I clean it all off. The boat looks beautiful again.... and two days later it looks like one of Alfred HitchCock's "The Birds" victims.
Oh woe is woe, what can be done??

Im in a marina as well but they seem to poop on the boats with masts rather than my rib (so far!). Suggest you park up between two yachts. Two other ribs moor close by with all-over-covers on them - Im thinking of doing the same thing - this would at least keep the guano off the tubes! Other boats use a big plastic owl or kestrel-type thing which works a bit. Some also use loads of thin string/thread spread over the boat from a post in the middle, but it takes ages to set up.

I have seen near where I am, that quite a lot of sail boat owners run a string down the length of the boat a foot or so off the deck and tie on CD's at about 12 inch intervals.

Now they are either particularly well heeled (obviously the bigger the boat the more cd's you can afford), recently and bitterly divorced (i.e. they were the ex-wife's favorite CD's) or have found a use for all those AOL and Freeserve CD's that seem to come with every magazine you buy.

I can't really say whether it truely works, but I haven't seen any birds roosting on them so far!